Implement a enableRequested signal for text-input-v3

The signal will be emitted if text-input-v3 applies a new enable state
if it's already enabled.

In Gtk's implementation, sending text_input_v3.enable for the second
time has the similar semantics of text-input-v2.showInputPanel. Try to
connect them together and add corresponding test.

See also: https://github.com/GNOME/gtk/blob/a95cfb1c457cbd3ff365f7f1244c42dda8540b66/gtk/gtkimcontextwayland.c#L555